Alloy Formation at the Tetrapod Core/Arm Interface

The nature of the interfacial structure between the core and the arms of a tetrapod quantum dot (QD) formed during the heteroepitaxial growth of a ZnS arm onto a CdSe core is not well understood but can be analyzed through the use of high-frequency electron paramagnetic resonance (HF-EPR) spectroscopy. The spectroscopic resolution at high frequency allows the presence of unique crystal fields reflecting interfacial alloying to be analyzed by incorporating Mn­(II) ions as a dopant into the QD to act as an intentional EPR active spectroscopic probe. In addition, the HF-EPR can spectroscopically observe the presence of ion vacancies that are anticipated to form at the heteroepitaxial interface to accommodate structural mismatch. The HF-EPR spectra for Mn­(II) are extremely sensitive to perturbations of the microenvironment due to changes in the crystal field. The HF-EPR spectra of Mn­(II) in a CdSe (core)/ZnS (arm) tetrapod exhibiting wurtzite symmetry for both core and interface of the tetrapod provide clear evidence of heteroalloying at the core–arm interface and formation of intrinsic dislocations at grain boundaries. The formation of the interfacial alloy and grain boundaries reflects short-range ion migration at the heteroepitaxial layer to reduce strain energy due to the 12% lattice mismatch between the wurtzite lattices of CdSe and ZnS

Synthesis and Characterization of a Stable High-Valent Cobalt Carbene Complex

The formally Co^IV carbene Co­(OR)₂(CPh₂) is formed upon the reaction of diphenyl­diazo­methane with the cobalt bis­(alkoxide) precursor Co­(OR)₂(THF)₂. Structural, spectroscopic, and theoretical studies demonstrate that Co­(OR)₂(CPh₂) has significant high-valent Co^IVCPh₂ character with non-negligible spin density on the carbene moiety

Synthesis and Characterization of a Stable High-Valent Cobalt Carbene Complex

The formally Co^IV carbene Co­(OR)₂(CPh₂) is formed upon the reaction of diphenyl­diazo­methane with the cobalt bis­(alkoxide) precursor Co­(OR)₂(THF)₂. Structural, spectroscopic, and theoretical studies demonstrate that Co­(OR)₂(CPh₂) has significant high-valent Co^IVCPh₂ character with non-negligible spin density on the carbene moiety

High Field Electron Paramagnetic Resonance Characterization of Electronic and Structural Environments for Paramagnetic Metal Ions and Organic Free Radicals in Deepwater Horizon Oil Spill Tar Balls

In the first use of high-field electron paramagnetic resonance (EPR) spectroscopy to characterize paramagnetic metal–organic and free radical species from tar balls and weathered crude oil samples from the Gulf of Mexico (collected after the Deepwater Horizon oil spill) and an asphalt volcano sample collected off the coast of Santa Barbara, CA, we are able to identify for the first time the various paramagnetic species present in the native state of these samples and understand their molecular structures and bonding. The two tar ball and one asphalt volcano samples contain three distinct paramagnetic species: (i) an organic free radical, (ii) a [VO]²⁺ containing porphyrin, and (iii) a Mn²⁺ containing complex. The organic free radical was found to have a disc-shaped or flat structure, based on its axially symmetric spectrum. The characteristic spectral features of the vanadyl species closely resemble those of pure vanadyl porphyrin; hence, its nuclear framework around the vanadyl ion must be similar to that of vanadyl octaethyl porphyrin (VOOEP). The Mn²⁺ ion, essentially undetected by low-field EPR, yields a high-field EPR spectrum with well-resolved hyperfine features devoid of zero-field splitting, characteristic of tetrahedral or octahedral Mn–O bonding. Although the lower-field EPR signals from the organic free radicals in fossil fuel samples have been investigated over the last 5 decades, the observed signal was featureless. In contrast, high-field EPR (up to 240 GHz) reveals that the species is a disc-shaped hydrocarbon molecule in which the unpaired electron is extensively delocalized. We envisage that the measured <i>g</i>-value components will serve as a sensitive basis for electronic structure calculations. High-field electron nuclear double resonance experiments should provide an accurate picture of the spin density distribution for both the vanadyl-porphyrin and Mn²⁺ complexes, as well as the organic free radical, and will be the focus of follow-up studies

Spin Dynamics and Relaxation in Graphene Nanoribbons: Electron Spin Resonance Probing

Here we report the results of a multifrequency (∼9, 20, 34, 239.2, and 336 GHz) variable-temperature continuous wave (cw) and X-band (∼9 GHz) pulse electron spin resonance (ESR) measurement performed at cryogenic temperatures on potassium split graphene nanoribbons (GNRs). Important experimental findings include the following: (a) The multifrequency cw ESR data infer the presence of only carbon-related paramagnetic nonbonding states, at any measured temperature, with the <i>g</i> value independent of microwave frequency and temperature. (b) A linear broadening of the ESR signal as a function of microwave frequency is noticed. The observed linear frequency dependence of ESR signal width points to a distribution of <i>g</i> factors causing the non-Lorentzian line shape, and the <i>g</i> broadening contribution is found to be very small. (c) The ESR process is found to be characterized by slow and fast components, whose temperature dependences could be well described by a tunneling level state model. This work not only could help in advancing the present fundamental understanding on the edge spin (or magnetic)-based properties of GNRs but also pave the way to GNR-based spin devices

Evidence of a ZnCr₂Se₄ Spinel Inclusion at the Core of a Cr-Doped ZnSe Quantum Dot

Herein we report doping of ZnSe by Cr ions leads to formation of small ZnCr₂Se₄ spinel inclusions within the cubic sphalerite lattice of a 2.8 nm CrZnSe quantum dot (QD). The Cr ion incorporates as a pair of Cr­(III) ions occupying edge-sharing tetragonal distorted octahedral sites generated by formation of three Zn ion vacancies in the sphalerite lattice in order to charge compensate the QD. The site is analogous to the formation of a subunit of the ZnCr₂Se₄ spinel phase known to form as inclusions during peritectoid crystal growth in the ternary CrZnSe solid-state compound. The oxidation state and site symmetry of the Cr ion is confirmed by X-ray absorption near edge spectroscopy (XANES), crystal field absorption spectroscopy, and electron paramagnetic resonance (EPR). Incorporation as the Cr­(III) oxidation state is consistent with the thermodynamic preference for Cr to occupy an octahedral site within a II–VI semiconductor lattice with a half-filled t_2g d-level. The measured crystal field splitting energy for the CrZnSe QD is 2.08 eV (2.07 eV form XANES), consistent with a spinel inclusion. Further evidence of a spinel inclusion is provided by analysis of the magnetic data, where antiferromagnetic (AFM) exchange, a Curie–Weiss (C–W) temperature of θ = −125 K, and a nearest-neighbor exchange coupling constant of <i>J</i>_NN = −12.5 K are observed. The formation of stable spinel inclusions in a QD has not been previously reported

Enhancing the Magnetic Anisotropy of Linear Cr(II) Chain Compounds Using Heavy Metal Substitutions

Magnetic properties of the series of three linear, trimetallic chain compounds Cr₂Cr­(dpa)₄Cl₂, <b>1</b>, Mo₂Cr­(dpa)₄Cl₂, <b>2</b>, and W₂Cr­(dpa)₄Cl₂, <b>3</b> (dpa = 2,2′-dipyridylamido), have been studied using variable-temperature dc and ac magnetometry and high-frequency EPR spectroscopy. All three compounds possess an <i>S</i> = 2 electronic ground state arising from the terminal Cr²⁺ ion, which exhibits slow magnetic relaxation under an applied magnetic field, as evidenced by ac magnetic susceptibility and magnetization measurements. The slow relaxation stems from the existence of an easy-axis magnetic anisotropy, which is bolstered by the axial symmetry of the compounds and has been quantified through rigorous high-frequency EPR measurements. The magnitude of <i>D</i> in these compounds increases when heavier ions are substituted into the trimetallic chain; thus <i>D</i> = −1.640, −2.187, and −3.617 cm^–1 for Cr₂Cr­(dpa)₄Cl₂, Mo₂Cr­(dpa)₄Cl₂, and W₂Cr­(dpa)₄Cl₂, respectively. Additionally, the <i>D</i> value measured for W₂Cr­(dpa)₄Cl₂ is the largest yet reported for a high-spin Cr²⁺ system. While earlier studies have demonstrated that ligands containing heavy atoms can enhance magnetic anisotropy, this is the first report of this phenomenon using heavy metal atoms as “ligands”

Synthesis, Detailed Characterization, and Theoretical Understanding of Mononuclear Chromium(III)-Containing Polyoxotungstates [Cr^III(HX^VW₇O₂₈)₂]^13– (X = P, As) with Exceptionally Large Magnetic Anisotropy

Two monochromium­(III)-containing heteropolytungstates, [Cr^III(HP^V­W₇O₂₈)₂]^13‑ (<b>1a</b>) and [Cr^III(HAs^V­W₇O₂₈)₂]^13‑ (<b>2a</b>), were prepared via simple, one-pot reactions in aqueous, basic medium, by reaction of the composing elements, and then isolated as hydrated sodium salts, Na₁₃[Cr^III(HP^V­W₇O₂₈)₂]·47H₂O (<b>1</b>) and Na₁₃[Cr^III(HAs^V­W₇O₂₈)₂]·52H₂O (<b>2</b>). Polyanions <b>1a</b> and <b>2a</b> comprise an octahedrally coordinated Cr^III ion, sandwiched by two {PW₇} or {AsW₇} units. Both compounds <b>1</b> and <b>2</b> were fully characterized in the solid state by single-crystal XRD, IR spectroscopy, thermogravimetric and elemental analyses, magnetic susceptibility, and EPR measurements. Magnetic studies on <b>1</b> and <b>2</b> demonstrated that both compounds exhibit appreciable deviation from typical paramagnetic behavior, and have a ground state S = ³/₂, as expected for a Cr^III ion, but with an exceptionally large zero-field uniaxial anisotropy parameter (<i>D</i>). EPR measurements on powder and single-crystal samples of <b>1</b> and <b>2</b> using 9.5, 34.5, and 239.2 GHz frequencies and over 4–295 K temperature fully support the magnetization results and show that <i>D</i> = +2.4 cm^–1, the largest and sign-assigned <i>D</i>-value so far reported for an octahedral Cr^III-containing, molecular compound. Ligand field analysis of results from CASSCF and NEVPT2-correlated electronic structure calculations on Cr­(OH)₆^3– model complexes allowed to unravel the crucial role of the second coordination sphere of Cr^III for the unusually large magnetic anisotropy reflected by the experimental value of <i>D</i>. The newly developed theoretical modeling, combined with the synthetic procedure for producing such unusual magnetic molecules in a well-defined and essentially magnetically isolated environment, appears to be a versatile new research area

Synthesis, Detailed Characterization, and Theoretical Understanding of Mononuclear Chromium(III)-Containing Polyoxotungstates [Cr^III(HX^VW₇O₂₈)₂]^13– (X = P, As) with Exceptionally Large Magnetic Anisotropy

Two monochromium­(III)-containing heteropolytungstates, [Cr^III(HP^V­W₇O₂₈)₂]^13‑ (<b>1a</b>) and [Cr^III(HAs^V­W₇O₂₈)₂]^13‑ (<b>2a</b>), were prepared via simple, one-pot reactions in aqueous, basic medium, by reaction of the composing elements, and then isolated as hydrated sodium salts, Na₁₃[Cr^III(HP^V­W₇O₂₈)₂]·47H₂O (<b>1</b>) and Na₁₃[Cr^III(HAs^V­W₇O₂₈)₂]·52H₂O (<b>2</b>). Polyanions <b>1a</b> and <b>2a</b> comprise an octahedrally coordinated Cr^III ion, sandwiched by two {PW₇} or {AsW₇} units. Both compounds <b>1</b> and <b>2</b> were fully characterized in the solid state by single-crystal XRD, IR spectroscopy, thermogravimetric and elemental analyses, magnetic susceptibility, and EPR measurements. Magnetic studies on <b>1</b> and <b>2</b> demonstrated that both compounds exhibit appreciable deviation from typical paramagnetic behavior, and have a ground state S = ³/₂, as expected for a Cr^III ion, but with an exceptionally large zero-field uniaxial anisotropy parameter (<i>D</i>). EPR measurements on powder and single-crystal samples of <b>1</b> and <b>2</b> using 9.5, 34.5, and 239.2 GHz frequencies and over 4–295 K temperature fully support the magnetization results and show that <i>D</i> = +2.4 cm^–1, the largest and sign-assigned <i>D</i>-value so far reported for an octahedral Cr^III-containing, molecular compound. Ligand field analysis of results from CASSCF and NEVPT2-correlated electronic structure calculations on Cr­(OH)₆^3– model complexes allowed to unravel the crucial role of the second coordination sphere of Cr^III for the unusually large magnetic anisotropy reflected by the experimental value of <i>D</i>. The newly developed theoretical modeling, combined with the synthetic procedure for producing such unusual magnetic molecules in a well-defined and essentially magnetically isolated environment, appears to be a versatile new research area

Coherent Spin Dynamics in Molecular Cr₈Zn Wheels

Controlling and understanding transitions between molecular spin states allows selection of the most suitable ones for qubit encoding. Here we present a detailed investigation of single crystals of a polynuclear Cr₈Zn molecular wheel using 241 GHz electron paramagnetic resonance (EPR) spectroscopy in high magnetic field. Continuous wave spectra are well reproduced by spin Hamiltonian calculations, which evidence that transitions in correspondence to a well-defined anticrossing involve mixed states with different total spin. We studied, by means of spin echo experiments, the temperature dependence of the dephasing time (<i>T</i>₂) down to 1.35 K. These results are reproduced by considering both hyperfine and intermolecular dipolar interactions, evidencing that the dipolar contribution is completely suppressed at the lowest temperature. Overall, these results shed light on the effects of the decoherence mechanisms, whose understanding is crucial to exploit chemically engineered molecular states as a resource for quantum information processing